To improve engine thermal efficiency and increase vehicle fuel economy, a lean burn engine approach has been used. In these systems, the engine is operated lean of stoichiometry and relatively unthrottled, thereby decreasing engine pumping work and further increasing fuel efficiency. To increase the range of lean operation, direct injection is used where fuel is injected directly into the engine. In particular, when operating in the stratified mode, which is always lean of stoichiometry, fuel is injected during a compression stroke. Alternatively, when operating in the homogeneous mode, which can be either rich or lean of stoichiometry, fuel is injected during an intake stroke.
Since the engine operates lean for extending periods, and is therefore outside the three way catalyst operating window, a lean NOx trap, or adsorbent, is used. The NOx trap adsorbs NOx when operating lean, and release NOx when operating at or rich of stoichiometry. The NOx trap can also adsorb sulfur, which reduces NOx storage capacity and therefor reduces system efficiency.
One method for removing sulfur requires raising the NOx trap temperature to elevated levels and operating the air/fuel ratio at or rich of stiochiometry. Various methods are known for elevating the NOx trap temperature. One particular method operates some cylinders lean and some cylinders rich. When these two exhaust streams meet at the NOx trap, an exotherm is created thereby heating the NOx trap. In this way, NOx trap temperature is elevated and sulfur can be removed thereby replenishing the NOx storage capacity. Such a system is described in U.S. Pat. No. 5,758,493.
The inventors herein have recognized a disadvantage with the above approach. For example, when some cylinders are operated rich and some are operated lean, there is a torque imbalance since all of the cylinders receive equivalent airflow. This torque imbalance, or fluctuation, causes increased vibration and degrades customer satisfaction. One method to reduce the torque imbalance uses retarded ignition timing in the rich cylinders. The inventors herein have recognized that when using ignition timing to solve the torque imbalance disadvantage, a degradation in fuel economy is experienced since ignition timing retard is used. In other words, fuel economy is reduced since the cylinders operating rich do not produce the full potential torque that could be produced if optimum ignition timing were employed.